Back motor running hot


my drone is designed in a way such that the back motors are mounted upside down on top frame while front motors are mounted normally. With this set up the back motors are always hotter compared to the front ones. Tried to place on the drone on the floor with 4 motors running, the results are still the same. All sensors are calibrated, CG is balanced and all remote sticks are centered.

Does anybody know what caused the back motor to be hotter than the front ones? thank you!